Article pubs.acs.org/IECR
Aerobic Oxidation of Cyclohexane Effectively Catalyzed by Simply Synthesized Silica-Supported Cobalt Ferrite Magnetic Nanocrystal Jinhui Tong,*,†,‡ Lili Bo,§ Xiaodong Cai,†,‡ Haiyan Wang,†,‡ Qianping Zhang,†,‡ and Lingdi Su†,‡ †
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education, Lanzhou, 730070 Gansu, P. R. China Key Laboratory of Gansu Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070 Gansu, P. R. China § College of Science, Gansu Agricultural University, Lanzhou, 730070 Gansu, P. R. China ‡
ABSTRACT: Silica-supported magnetic cobalt ferrite complex oxides, CoFe2O4/SiO2, with different loading of 5, 10, 20, and 50% were simply prepared by a sol−gel autocombustion method using colloidal aqueous silica solution as a cheap silica source. The as-prepared samples were well characterized by X-ray diffractometry (XRD), Fourier transform infrared spectrophotometry (FT-IR), transmission electron microscopy (TEM), and N2 physisorption. Metals contents of the samples were also determined by atomic absorption spectrophotometry. Their catalytic performances were evaluated on cyclohexane oxidation using oxygen as oxidant in the absence of solvents and reductants. The supported catalysts have shown high catalytic activities for cyclohexane oxidation. Especially, when 5% loading of CoFe2O4/SiO2 was employed, 4181 turnover number and 95.4% selectivity for cyclohexanone and cyclohexanol were obtained under 1.6 MPa of initial oxygen pressure at 418 K after 6.0 h of reaction. The sample of 50% of CoFe2O4/SiO2 has strong magnetism and can be magnetically separated easily. This sample was employed as catalyst to optimize the reaction conditions. The catalyst showed prominent reusability, and no obvious loss in activity was observed when reused in six consecutive runs.
1. INTRODUCTION Selective oxidation of saturated hydrocarbons is one of the major challenges in catalysis chemistry.1,2 Especially, the selective oxidation of cyclohexane into cyclohexanol and cyclohexanone is of considerable importance in the chemical industry for producing Nylon-6 and Nylon-66 polymers.3,4 Unfortunately, this reaction is proved to be one of the least efficient industrial processes.5 The present industrial process for cyclohexane oxidation is limited by low cyclohexane conversion of 3−6% and selectivity of 75−80%.6 Considerable catalytic systems for cyclohexane oxidation have been reported in recent years.7−14 However, most of the current processes are limited by using toxic solvents and/or reductants as well as harsh reaction conditions,7−10,15,16 so more efficient and environmentally friendly catalytic processes are desired, especially heterogeneous ones using molecular oxygen as oxidant without addition of solvents or reductants. Transition metal oxides are a kind of important catalyst for the oxidation of hydrocarbons.17−19 Recently, many transition metal oxides have been used to catalyze oxidation of hydrocarbons.20−24 Especially, nanosized materials have attracted much attention in recent years due to their wellknown unique physical and chemical properties.19,25 A variety of nanosized catalysts have been developed for cyclohexane oxidation in the past decades.19,25−27 Nanosized oxides, such as Fe2O3, Co3O4, and mixed Fe−Co oxide, have been employed as catalysts for cyclohexane oxidation.5,25,28 In order to obtain some desired properties, such as high dispersibility, stability, and activity, supported nanosized oxides catalysts on inert or inactive supports have been intensively pursued from both fundamental and applied aspects.29 Recently, many examples have been reported for cyclohexane oxidation catalyzed by © 2014 American Chemical Society
supported metals and/or metal oxides catalysts prepared with diverse methods, such as cobalt−silicon mixed oxide nanosphere prepared with a modified reverse-phase microemulsion method,29 Au/MxOy/Al2O3 (M = Co, Zr, and Ce) catalysts prepared by an impregnation-ammonia washing method,30 Au/ Co3O4 catalysts prepared by a coprecipitation method,31 Pt/ oxide (Al2O3, TiO2, and ZrO2) catalysts prepared by impregnation, MgO-supported vanadium oxide catalysts prepared by wet impregnation and grafting,32 Ni-clay catalysts prepared by impregnation,33 and Cr−Si mixed oxides prepared by hydrolysis.34 Unfortunately, these catalysts systems are limited by complex preparation processes, employment of noble metals, and a coreductant or toxic agent, and most of them are of low efficiency. In our previous work,35 we found that cobalt ferrite spinel prepared by a modified sol−gel autocombustion route as active phase has shown higher catalytic activity on aerobic oxidation of cyclohexane than cobalt and iron oxides as well as their mechanical mixture free of solvents and reducing agents. The preparation method is generally preferred due to inexpensive precursors, short preparation time, modest heating, and relatively simple manipulations.36−38 The catalyst can be separated easily by external magnetic field and exhibited excellent reusability. However, it is well-known that magnetic nanoparticles are prone to agglomerate due to both of their large surface energy and strong magnetic interactions.36 In order to improve the Received: Revised: Accepted: Published: 10294
October May 12, May 14, May 14,
30, 2013 2014 2014 2014
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Agilent 6820 gas chromatograph. After the decomposition of cyclohexyl hydroperoxide (CHHP) to cyclohexanol by adding triphenylphosphine to the reaction mixture, cyclohexanone (K) and cyclohexanol (A) were determined by the internal standard method using toluene as internal standard. The concentration of CHHP was determined by iodometric titration. The molar amount of cyclohexanol was calculated by subtracting the fraction by decomposition of CHHP from the detected amount by GC. The main byproducts of the reaction are CHHP, hexanedioic acid, hexanoic acid, dicyclohexyl adipate, and cyclohexyl caproate, and trace of CO2 were detected in all the cases (Scheme 1). Carbon balance was checked to quantify
dispersibility and catalytic activity of the catalyst, in this work, highly dispersed CoFe2O4/SiO2 catalysts with different CoFe2O4 loading were prepared by the simple method mentioned above using colloidal silica as a cheap SiO2 source. The catalytic performances of the samples in cyclohexane oxidation were investigated using molecular oxygen as oxidant. Compared with pristine CoFe2O4, catalytic activities of CoFe2O4/SiO2 samples were greatly improved, and nearly ten times higher turnover numbers were obtained under more mild conditions.
2. EXPERIMENTAL SECTION 2.1. Materials and Equipment. Cyclohexane and toluene were purified before use. Other reagents were of analytical grade and were used as received. FT-IR spectra were measured on a Nexus 870 FT-IR spectrophotometer. XRD patterns of the samples were collected using a PANalytical X’Pert Pro diffractometer with Cu−Kα radiation. TEM micrographs were obtained using a Hitachi H-600 microscope. The BET surface area measurements were performed on a Micromeritics ASAP 2010 instrument at liquid nitrogen temperature. Metal contents of the samples were determined on a Hitachi 180-80 polarized Zeeman atomic absorption spectrophotometer. The oxidation products were determined by a HP 6890/5973 GC/MS instrument and quantified by an Agilent 6820 gas chromatograph using toluene as internal standard. 2.2. Preparation of CoFe2O4/SiO2 Catalysts. The silicasupported cobalt ferrite complex oxides were prepared as follows. First, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, and citric acid were completely dissolved in distilled water with a 1:1.5 molar ratio of metal (CoII + FeIII) to citric acid. Next, a calculated amount of colloidal aqueous silica solution (SiO2, 40.0 wt %, Guangzhou Renmin Chemical Plant, China) was added to the solution, and then concentrated ammonia (25− 28%) was added slowly under constant stir to adjust the pH to neutral. The mixture was allowed to evaporate in an oil bath under continuous stirring at 80−90 °C until a gel formed. After the reaction, the formed gel was dried at 110 °C, and a xerogel was obtained. Finally, the produced xerogel was ignited at 650 °C, a self-propagating combustion process occurred, and a dark gray product was obtained after it combusted completely. Four samples with different calculated CoFe2O4 loadings of 5, 10, 20, and 50% were prepared as described above and designated as CFO/SiO2-5, CFO/SiO2-10, CFO/SiO2-20, and CFO/SiO250, respectively. The samples were pulverized finely in an agate mortar and then used to catalyze the oxidation of cyclohexane with molecular oxygen. 2.3. Oxidation of Cyclohexane. The oxidation of cyclohexane was performed in a 30 mL stainless steel autoclave equipped with Teflon coated magnetic stirrer and an automatic temperature controller. Typically, 5.0 mg of catalyst and 7.0 mL (65.3 mmol) of cyclohexane were added to the autoclave. The autoclave was flushed three times with O2, pressurized to the desired pressure, and then heated to the desired temperature with stirring. After the reaction, the autoclave was cooled to room temperature. The gas-phase mixture was collected and analyzed by gas chromatography (GC) equipped with a 5A molecular sieve column and a thermal conductivity detector (TCD). Liquid-phase aliquots were identified by GC-MS and quantified according to the following reported method.28,29 The reaction mixture was diluted with 15.00 g of ethanol to dissolve the byproducts. The reaction products were identified by a HP 6890/5973 GC/MS instrument and quantified by an
Scheme 1. Oxidation of Cyclohexane
unidentified products and coke deposition and was defined as follows: carbon balance (%) = 100 − [mol(A + K + CHHP)] /[(mol cyclohexane)in − (mol cyclohexane)out ] × 100
3. RESULTS AND DISCUSSION 3.1. Characterization of the Catalysts. The compositions of the samples were primarily recognized through the XRD measurement, and the patterns are shown in Figure 1. For all
Figure 1. XRD patterns of the as-prepared samples.
the samples, the broad and diffuse diffraction peak at around 2θ = 21.8° is attributed to amorphous silica. In XRD patterns of the samples CFO/SiO2-20 and CFO/SiO2-50, seven peaks at 30.2, 35.6, 37.2, 43.2, 53.1, 57.2, and 62.7° can be ascribed to the reflection of (220), (311), (222), (400), (422), (511), and (440) diffractions of CoFe2O4 (JCPDS no. 22-1086), showing the presence of spinel ferrite phase (Figure 1). It also shows that minor Fe1.6SiO4 coexists with the spinel ferrite in these two samples. 10295
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Figure 2 shows FT-IR spectra of the five samples. The typical bands of silica gel at about 1100, 810, and 475 cm−1 are present
vibrations of Si−O−Fe, and it reflects some interaction between the highly isolated Fe3+ ions and the nearest silica matrix; the band at 580 cm−1 can be ascribed to the Fe−O stretching vibration of Si−O−Fe, and it further confirms the existing of Si−O−Fe.40 The TEM micrographs of the samples with different scales are shown in Figure 3. The morphology of sample CFO/SiO25 is very similar to that of CFO/SiO2-10 and therefore was not displayed tediously. It can be seen that all the samples show spheric nanoparticles broadly distributed from 15 to 20 nm to 35−45 nm, with a high percentage of small particles (25−35 nm). For samples prepared by the sol−gel autocombustion method, the nanoparticles become more agglomerative with an increase of cobalt ferrite loading (Figure 3). Metals contents, BET surface areas, and the total pore volumes data of the samples were summarized in Table 1. The molar ratio of Co to Fe in all the samples is 1:2. For the samples prepared with the sol−gel autocombustion route, BET surface area and total pore volume decreased with an increase of metal loading due to more cobalt ferrite filling in the piled pores of the samples and increasing magnetic agglomeration consequently. The sample CFO/SiO2-AC-5 has higher BET surface area and total pore volume than other samples. This should be attributed to comparatively higher dispersion of the particles in the sample CFO/SiO2-AC-5. 3.2. Catalysis Tests. The catalytic activities of the supported catalysts on cyclohexane oxidation were investigated under the optimum reaction conditions in our previous work, and the results are listed in Table 2, and those over other
Figure 2. FT-IR spectra of the samples.
for all the samples, and it confirms expected formation of the silica network.39,40 Two weak absorption bands around 3420 and 1630 cm−1 can be ascribed to stretching and bending vibrations of O−H in adsorbed water. For the sample CFO/ SiO2-50, the bands at 960 and 860 cm−1 are associated with
Figure 3. TEM morphology of (a-c): CFO/SiO2-10; (d-f): CFO/SiO2-20; and (g-k): CFO/SiO2-50. 10296
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amount, initial oxygen pressure, reaction temperature, reaction time, and the reusability of the catalyst were investigated. 3.3. Effect of Catalyst Amount. The catalyst amount was optimized, and the results are listed in Table 3. It is obvious that an increase of the catalyst amount is in favor of cyclohexane conversion. As for the products distribution, with an increase of the catalyst amount, selectivity of cyclohexanol and the total selectivity for cyclohexanol and cyclohexanone decreased, while the selectivity of cyclohexanone changed slightly. This confirms that more byproducts formed with an increase of the catalyst amount. As a result, carbon balance increased from 6.7 to 10.0% when the catalyst amount increased from 3.0 to 9.0 mg. Considering both cyclohexane conversion and the total selectivity for cyclohexanol and cyclohexanone, 5.0 mg was selected as the optimum catalyst amount. 3.4. Effect of Initial Oxygen Pressure. Effect of initial oxygen pressure on conversion and selectivities of the products was investigated in a range of 0.8−2.0 MPa. The results are shown in Figure 4. As expected, cyclohexane conversion increased expectedly from 5.2 to 10.3% with an increase in the initial oxygen pressure from 0.8 to 2.0 MPa. As for the products distribution, with increasing initial oxygen pressure from 0.8 to 2.0 MPa, the selectivity for cyclohexanol decreased from 42.0 to 21.1% while that for cyclohexanone increased from 54.7 to 67.8%. This indicates that cyclohexanol is apt to be further oxidized to cyclohexanone at high oxygen pressure. As for CHHP, which is an unstable intermediate and could decompose to cyclohexanol/cyclohexanone,28 the distribution of it showed a similar trend with that of cyclohexanol. Only 0.5% of CHHP was obtained at 0.8 MPa, and almost no CHHP was detected when the initial oxygen pressure was above 1.4 MPa. The selectivity for K/A oil decreased all along from 96.7 to 88.9% with an increase of initial oxygen pressure due to more deep oxidation products formed under higher oxygen pressure. As a result, the carbon balance increased from 2.8 to 11.1% when the initial oxygen pressure was raised from 0.8 to 2.0 MPa. 3.5. Effect of Reaction Temperature. Figure 5 shows the results of cyclohexane oxidation at different temperatures. It is
Table 1. Metals Content, BET Surface Area ,and Total Pore Volume of the Samples catalyst CFO/ SiO2AC-5 CFO/ SiO2AC-10 CFO/ SiO2AC-20 CFO/ SiO2AC-50
Co content (mmoL/g)
Fe content (mmoL/g)
BET surface area (m2/g)
total pore volume (cm3/g)
0.231
0.457
91.8
0.43
0.453
0.906
91.1
0.41
0.874
1.749
86.1
0.30
1.687
3.355
66.9
0.20
catalysts are also shown for comparison. To investigate the catalytic effect of stainless steel autoclave itself, the reaction with no catalyst was also carried out, and the results were listed as entry 5. The results over SiO2 support and pure CoFe2O4 were also listed for comparison (Table 2, entries 6 and 7). It is clear that, in the absence of ferrite (entries 5 and 6), only a low conversion and selectivity for K/A oil were obtained. By comparison with the pristine CoFe2O4 (entry 7), the activities of supported catalysts were greatly improved, and much higher turnover numbers were obtained. The total selectivity for K/A oil also increased slightly. For the catalysts prepared by the sol− gel autocombustion method, with an increase of cobalt ferrite loading, cyclohexane conversion increased, but the turnover number decreased. Excitedly, the sample CFO/SiO2-AC-5 obtained the highest turnover number of 4181, which is much higher than the reported Co3O4/Al2O3; even the noble metal contained Au/Co3O4/Al2O3 and Au/Co3O4 catalysts under comparable conditions. Although the highest turnover number was obtained on the sample CFO/SiO2-AC-5 and the carbon balance was increased with the loading of the catalyst, only the sample CFO/SiO2-50 can be separated effectively by external magnetic field. In pursuit of the convenient magnetic separation, the sample CFO/SiO2-50 was selected as catalyst to optimize the reaction conditions, and the effects of catalyst Table 2. Oxidation of Cyclohexane with Different Catalystsa
selectivity (mol %) entry
catalyst
conversion (mol %)
TONb
A
K
A+K
CHHP
carbon balance (%)
1 2 3 4 5 6 7 8 9 10
CFO/SiO2-5 CFO/SiO2-10 CFO/SiO2-20 CFO/SiO2-50 − SiO2c CoFe2O4d Co3O4/Al2O3e Au/Co3O4/Al2O3e Au/Co3O4h
7.4 8.2 9.1 9.9 3.2 3.5 13.7 6.4 9.5 8.8
4181 2360 1358 765 − − 426 2000f 2379g 3351i
42.3 42.3 40.2 23.5 35.2 34.9 33.4 48.1 52.5 51.1
53.1 52.4 53.2 69.1 51.0 51.8 60.5 33.0 40.3 40.4
95.4 94.7 93.4 92.6 86.2 86.7 93.9 81.1 92.8 91.5
trace 0 0 0 11.2 10.3 0 11.3 1.79 0.79
4.6 5.3 6.6 7.4 2.6 3.0 6.1 − − −
a
Reaction conditions: cyclohexane 7.0 mL (65.3 mmol); catalyst 5.0 mg; initial oxygen pressure = 1.6 MPa; temperature = 418 K; reaction time = 6.0 h. bTON = moles of substrates converted per mol of CoFe2O4 loaded. cFrom colloidal aqueous silica solution and prepared by self-propagating combustion process without addition of metals. dPristine CoFe2O4 was synthesized according to the procedure described in our previous work.35 e Results from ref 30. Reaction conditions: cyclohexane 20.0 mL; catalyst 50.0 mg; initial oxygen pressure = 1.5 MPa; temperature = 423 K; reaction time = 3.0 h. fTON = moles of substrates converted per mol of Co3O4 loaded, is calculated by ourselves based on the data from the reference. gTON = moles of substrates converted per mol of Au and Co3O4 loaded is calculated by ourselves based on the data from the reference. hResults from ref 31. Reaction conditions: cyclohexane 20.0 mL; catalyst 50.0 mg; initial oxygen pressure = 1.5 MPa; temperature = 423 K; reaction time = 3.0 h. i TON = moles of substrates converted per mol of Au loaded is calculated by ourselves based on the data from the reference. 10297
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Table 3. Effect of Catalyst Amount on Oxidation of Cyclohexanea selectivity (mol %) catalyst amount (mg)
conversion (mol %)
TONb
A
K
A+K
CHHP
carbon balance (%)
3.0 5.0 7.0 9.0
7.2 9.9 11.2 12.1
927 765 618 519
25.8 23.5 22.2 21.3
67.5 69.1 69.0 68.7
93.3 92.6 91.2 90.0
trace 0 0 0
6.7 7.4 8.8 10.0
a Reaction conditions: cyclohexane 7.0 mL (65.3 mmol); catalyst CFO/SiO2-50; initial oxygen pressure = 1.6 MPa; temperature = 418 K; reaction time = 6.0 h. bTON = moles of substrates converted per mol of CoFe2O4 loaded.
temperature increased from 130 to 160 °C. Distribution of CHHP also decreased with reaction temperature, and 0.6% of CHHP was obtained at 130 °C; almost no CHHP was detected when the temperature was above 135 °C because of its decomposition. Additionally, the selectivity for K/A oil decreased from 97.1 to 88.2% due to more byproducts formed at higher temperature. As shown in Figure 5, the carbon balance increased from 2.3 to 11.8% when the reaction temperature increased from 130 to 145 °C. 3.6. Effect of Reaction Time. Figure 6 presents the effect of reaction time on cyclohexane oxidation. It can be seen clearly
Figure 4. Effect of initial oxygen pressure on cyclohexane oxidation. Reaction conditions: cyclohexane 7.0 mL (65.3 mmol); catalyst 5.0 mg; temperature = 418 K; reaction time = 6.0 h.
Figure 6. Effect of reaction time on cyclohexane oxidation. Reaction conditions: cyclohexane 7.0 mL (65.3 mmol); catalyst 5.0 mg; initial oxygen pressure = 1.6 MPa; temperature = 418 K.
that almost no cyclohexane was oxidized during the first 2 h, which might be the initiation period according to the generally accepted free radical mechanism.41 After this period, cyclohexane conversion increased rapidly from 0.7 to 8.6% in an additional 3 h and reached 10.3% after 7 h of reaction. With prolonging of the reaction time, the selectivity for cyclohexanone increased first and then decreased slowly, and the maximum value of 69.1% was obtained for 6 h of reaction. The selectivity for cyclohexanol decreased all along with an increase of reaction time from 49.8 to 22.1% after 7 h of reaction, because cyclohexanol is oxidized to cyclohexanone and other deep oxidation products. It can also be seen that the total selectivity for cyclohexanol and cyclohexanone decreased with prolonging of the reaction time due to more byproducts formed. It is also clear that the selectivity for cyclohexanol and cyclohexanone decreased from 98.5 to 90.3% when the reaction time was prolonged from 1 to 7 h, while the carbon balance increased up to 9.7%. As for the distribution of CHHP, it decreased from 1.5% to zero after 5 h of reaction.
Figure 5. Effect of reaction temperature on cyclohexane oxidation. Reaction conditions: cyclohexane 7.0 mL (65.3 mmol); catalyst 5.0 mg; initial oxygen pressure = 1.6 MPa; reaction time = 6.0 h.
clear that high temperature is in favor of cyclohexane conversion. Especially, when the reaction temperature increased from 130 to 145 °C, the cyclohexane conversion increased sharply from 4.3 to 9.9%; however, only 2.6% of an increase was shown when the temperature further increased to 160 °C. With an increase of temperature, the selectivity for cyclohexanol decreased, and the molar ratio of K/A oil increased, probably due to cyclohexanol being further oxidized to cyclohexanone. As can be seen from Figure 5, the selectivity for cyclohexanol decreased from 33.3 to 18.0% while that for cyclohexanone increased from 63.8 to 70.2% when the 10298
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3.7. Reuse of the Catalyst. After the reaction, the mixture was poured into a flask, and most of the catalyst can be seen being adsorbed on the magnetic stirrer. The catalyst together with the magnet can be easily separated by simple decantation after applying a magnetic field on the surface of the flask (Figure 7) was incorporated with the residual catalyst separated
precursor is cheap, and the method is effective and applicable. Comparisonly, the samples have shown greatly improved activities than the pristine CoFe2O4 under more mild reaction conditions, and high turnover numbers and selectivity of K/A oil can be obtained. The sample loading 5% of cobalt ferrite has shown optimum catalytic activity, 95.4% of selectivity for K/A oil and as high as 4184 of turnover number were obtained after 6 h of reaction. When the loading of cobalt ferrite is over 50%, the sample can be magnetically separated and exhibited excellent reusability. The samples showed high efficiency and good application prospect.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-931-7970359. Fax: +86-931-7970359. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 7. Photographs of magnetic separation of catalyst within 1 min.
ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (51302222, 21363021), Natural Science Foundation of Gansu Province (1308RJYA017), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1177) for financial support.
by centrifugation and then subjected to the second run under the same conditions. The data obtained were depicted in Figure 8. The selectivity for K/A oil changed slightly after six runs, and
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REFERENCES
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Figure 8. Reuse of the catalyst. Reaction conditions: cyclohexane 7.0 mL (65.3 mmol); catalyst 5.0 mg; initial oxygen pressure = 1.6 MPa; temperature = 418 K; reaction time = 6.0 h.
the average of 91.5% was obtained while cyclohexane conversion dropped from 9.9 to 7.6% with the average of 8.5%. The decrease in cyclohexane conversion could be mainly attributed to the unavoidable loss of the catalyst during the process of collection. After the reaction, the solution was analyzed for cobalt and iron using atomic absorption spectrophotometry. The results indicated that less than 0.08% of the stating metals had leached out after six runs. Based on the above results, it can be concluded that the supported cobalt ferrite has good stability and recyclability for aerobic oxidation of cyclohexane under our reaction conditions.
4. CONCLUSIONS Silica-supported cobalt ferrite spheric nanoparticles were prepared by a simple sol−gel autocombustion route using colloidal silica as silicon source without calcination. The 10299
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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on June 10, 2014. The heading for the first column of Table 3 has been modified. The correct version was published on June 11, 2014.
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dx.doi.org/10.1021/ie5008213 | Ind. Eng. Chem. Res. 2014, 53, 10294−10300
